1. Field of the Invention
The present invention relates to a power conversion device that converts DC power to AC power, or AC power to DC power.
2. Description of Related Art
In recent years, various types of environmentally-sensitive automobiles such as electric vehicles and hybrid electric vehicles employing an electric motor as a power source have become widespread. The main characteristic of such an environmentally sensitive automobile is that it is equipped with a battery, and that its wheels are driven by torque generated by a motor that utilizes this battery as a source of power. Generally an AC electric motor such as a permanent magnet synchronous electric motor or the like is used as this driving electric motor, since this makes it possible to make the power train more compact. Furthermore, a power conversion device that converts the DC power from the battery to AC power is utilized in order to supply the DC power from the battery to such an AC electric motor. By controlling this power conversion device, it becomes possible to perform variable speed change control of the AC electric motor for powering the vehicle.
In the drive system including the electric motor, the power conversion device, and the battery that is used in this type of hybrid electric vehicle, in order for the power conversion device to fulfill its power conversion function, it includes a power conversion device circuit that includes switching devices. Power conversion from DC power to AC power, or from AC power to DC power, is performed by these switching devices repeatedly going continuous and discontinuous.
When mounted to the vehicle, such a power conversion device receives DC power from the onboard power supply, and, for example, converts this DC power into three phase AC power for supply to a rotating electrical machine for driving the vehicle (i.e., the motor). Now, in recent years, the torque that is requested from such a rotating electrical machine for vehicle driving has become much greater than was the case with earlier device configurations. Due to this, there is a tendency for the amount of power to be converted by the power conversion device to become greater. Furthermore, a power conversion device that is mounted to a vehicle is subjected to a higher temperature environment, as compared with a power conversion device that is mounted to a typical industrial machine that is installed in a workplace. Due to this, with a power conversion device for a vehicle, it is desirable to emphasize as much as possible reduction of the amount of heat generated by the power conversion device itself, as compared with the case of a power conversion device for general use. The heat that is generated by the switching devices incorporated in the power conversion device circuitry is a large proportion of the heat that is generated internally to the power conversion device as a whole. Due to this, it is desirable to reduce the heat generated in the switching devices as much as possible.
Two reasons for generation of heat in the switching devices are steady loss that takes place even while the switching devices are in the continuous state, and switching loss that takes place when the switching devices change between the discontinuous state and the continuous state. While it is difficult to reduce the amount of heat generated due to steady loss because it is determined by the load that is being output, it is possible to reduce the amount of heat generated due to switching loss by reducing the number of times that switching is performed per unit time.
Attempts to reduce switching loss have been proposed in the prior art. For example, there is a per se known method of two phase modulation for reducing the switching loss of a three phase inverter. Since with such a two phase modulation method, in accordance with its name, switching control is performed for only two of the three phases while the remaining one phase is not switched, accordingly, as compared with the sine wave PWM method, the number of times that switching is performed is approximately ⅔, so that the switching loss is ⅔ or less that of sine wave PWM.
In order to reduce the switching loss further, it is necessary to lower the carrier frequency that determines the switching frequency, but when the carrier frequency is decreased the current pulsation increases, and along with this the torque pulsation of the motor and the noise that it generates become greater. Accordingly in the prior art methods have been investigated for suppressing undesirable current pulsations while reducing the carrier frequency (for example, refer to Patent Document #1).
With the method described in Patent Document #1, PWM switching operation is determined by comparing together the magnitude of the modulated signal and the magnitude of a reference signal that is the desired value for the output voltage. At this time, in order to reduce torque pulsation of the motor, at least one harmonic component is intentionally superimposed upon the reference signal, and thereby torque pulsation at a specific frequency may be eliminated or reduced. However, sometimes it becomes difficult to reduce the torque pulsation as desired when the superimposed harmonic component and the carrier frequency approach one another, and on the contrary there is a possibility that the torque pulsation may increase.
Patent Document #1: Japanese Laid-Open Patent Publication 62-239868
Thus, an object of the present invention is to provide a power conversion device, with which it is possible to reduce torque pulsation and switching losses.